CE-MRA is a technique used in the field of MRI. In CE-MRA, a contrast agent (also referred to as angiography agent) is injected to a vessel to enhance the contrast of a nuclear magnetic resonance image to obtain a clear and visual image of the vessel.
Ideally, a CE-MRA scan is triggered (started) when the concentration of the contrast agent reaches a peak value in a Region of Interest (ROI). However, in fact, there is a time period from when the CE-MRA scan is triggered to when an image of the ROI is acquired by the CE-MRA scan.
In order to get an optimal trigger timing of the CE-MRA scan, a monitoring scan is performed to observe the flow of the contrast agent. As blood is flowing, a monitoring scan region (also called a monitor region) is generally arranged with an offset away from a CE-MRA scan region in the direction of the blood flow, as shown in FIG. 1. The monitor region is located in the same plane with the CE-MRA scan region so as to ensure that the monitor region is identical to the scan region or at least includes a part of the scan region. By arranging the monitor region to lead the scan region by the offset, an operator of a CE-MRA system is allowed to have enough time to trigger the CE-MRA scan when seeing the contrast agent in a fluoroscopic image (a monitor image) acquired from the monitoring scan.
U.S. Pat. No. 6,489,486B1 discloses a Magnetic Resonance (MR) pre-imaging method. In this U.S. patent application, a monitor region is located manually. In addition, the operator of an MR system must focus on the continuously-displayed fluoroscopic images so as to observe the flow of the contrast agent and determine a trigger timing for a CE-MRA scan. Therefore, the operator must be experienced.
Several methods have been developed in the prior art to trigger a CE-MRA scan automatically. U.S. Pat. No. 6,167,293A discloses a method for performing MRA. In this U.S. patent application, a signal value, i.e. the concentration of a contrast agent, in a pre-selected region, i.e. a monitor region, is monitored, and a CE-MRA scan is started automatically when the signal value exceeds a specified threshold value. In this method, the monitor region is also manually selected by an operator. Moreover, in this method, the advance or delay of the contrast agent peak due to the blood flow velocity is not taken into consideration.